Electromagnetic motor rotatable in either direction

ABSTRACT

An electromagnetic stepping motor has a stator with an armature having the shape of an isosceles trapezoid the base of which is interrupted by a central gap and which is provided with three pole faces. The rotor comprises a permanent magnet. The stator comprises two coils one of which is located between a pole face opposite said gap and one of the other two pole faces. The other coil is located between the pole face opposite said gap and the other of said other two pole faces. When a current passes through the coils, the rotor is subjected to magnetic fields having directions which are oblique to each other and which are symmetrical with respect to a diameter of the rotor. The sense of the coil currents determines the sense of the fields. The arrangement is such that one can create in the zone of the rotor a resultant magnetic field which can extend in any one of four different directions dependent on the sense of the currents passing through the coils. By appropriate commutation of the sense of each of the two coil currents, one can cause the resultant magnetic field to rotate in one direction or in the other, to drive the rotor in one or the other directional sense, but always in the same direction for a given field rotation. Thus, the rotor rotates always in the desired direction, even if a rotational step is missed or if the rotor makes one rotational step too many.

This application is a division, of application Ser. No. 6,165,563, filedJuly 3, 1980.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an electromagnetic stepping motor operable ineither direction of rotation.

2. Description of the Prior Art

French Pat. No. 2,209,251, for example, illustrates such a motorcomprising two coils which are activated one after the other to producerotation of the rotor of the motor in one directional sense or theother, by steps each of 180°. Each coil must be dimensioned so as tofurnish, by itself, the energy necessary for this rotation, that is tosay each coil must have the same volume as the coil of a conventionalstepping motor which can rotate in only one directional sense.

Swiss Patent Application Ser. No. 10,768/71 illustrates a stepping motoroperable in either direction of rotation and comprising only one coil,but the rotor in this case rotates by 360° at each step. This is adrawback from the point of view of the mechanical arrangement since thestep-down ratio between the motor and the members it drives is ofimportance.

U.S. Pat. No. 4,112,671 illustrates a stepping motor operable in eitherdirection of rotation which comprises only one coil and the rotor ofwhich rotates by only 180° at each step. An electronic circuit controlsthe rotation in one directional sense or the other. This type of motorhas the serious drawback however that, if a step is by chance missed orif the motor makes one step too many, the direction of rotation isreversed.

SUMMARY OF THE INVENTION

The object of this invention is to overcome these drawbacks whilefurnishing an electromagnetic stepping motor operable in eitherdirection of rotation and the rotor of which rotates by 180° for eachstep always in the desired sense, even after a step has been missed orafter a step too many has been made. The motor has two coils which aresimultaneously and not alternatively activated; consequently, thesecoils have a total volume which is substantially equal to the volume ofthe single coil of a uni-directional stepping motor.

This object is achieved by the present invention with a stator of themotor being arranged to subject the rotor to two magnetic fieldsrespectively produced by two coils of the stator. The directions ofthese magnetic fields are oblique and substantially symmetrical withrespect to a diameter of the rotor, or more generally with respect to astraight line intersecting the axis of rotation of the rotor.Preferably, the two magnetic fields are applied to the rotor by threeenlarged pole faces surrounding the rotor, one of the said pole facesbeing common to the two coils and the other two respectively associatedwith each of the coils.

Other features of the invention will be apparent from the followingdescription, drawings and claims, the scope of the invention not beinglimited to the drawings themselves as the drawings are only for thepurpose of illustrating ways in which the principles of the inventioncan be applied. Other embodiments of the invention utilizing the same orequivalent principles may be used and structural changes may be made asdesired by those skilled in the art without departing from the presentinvention and the purview of the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 to 4 diagrammatically illustrate a motor representing a firstembodiment of the invention, this motor being respectively shown in thefour configurations of its operation;

FIG. 5 is a diagram illustrating graphically the energizing currentpulses passing through stator coils of the motor of FIGS. 1 to 4;

FIG. 6 shows a supply circuit of the coils of the motor;

FIGS. 7a and 7b respectively comprise tables summarizing the operationof this motor;

FIG. 8 diagrammatically illustrates a second embodiment of theinvention;

FIG. 9 is a view from above illustrating a third embodiment of theinvention;

FIG. 10 is a sectional view, on the line X--X of FIG. 9, of said thirdembodiment of the invention;

FIG. 11 is a plan view of a fourth embodiment of the invention;

FIGS. 12 and 13 are vertical sectional views, taken respectively on thelines XII--XII and XIII--XIII of FIG. 10, of the fourth embodiment; and

FIG. 14 is a view of a detail of FIGS. 8 and 10 showing a modificationof pole faces thereof.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The motor illustrated in FIGS. 1 to 4 comprises a stator 1 constitutedby a member made of soft magnetic material presenting the generalconfiguration of an isosceles trapezoid the base of which is interruptedwith a gap provided by a slit at 2a. The two ends of this memberconstitute two arcuate pole faces one of which is designated 1a and theother 1b, while the portion opposed to the slit 2a provides an arcuatepole face 1c. These three pole faces are positioned, in this example, atan angular spacing of substantially 120° with respect to each other,about a point 3 constituting the center of a rotor 4 of the motor. Thesethree pole faces further define two other slits, designated 2b and 2c.The rotor comprises a permanent magnet the poles of which arediametrically opposed and are designated N and S. The arcuate pole faces1a, 1b and 1c each extend over an angular zone slightly less than 120°in the example illustrated. However, the angular zones occupied by eachof the poles faces could be substantially changed depending on thedesired characteristics of the motor, its dimensions or the materialschosen for its construction. In any case, the angular zones occupied bythe two pole faces 1a and 1b are substantially equal. The pole faces 1aand 1b have a shape so that the air gap between them and the rotor 4 hasa variable width. This air gap is a minimum in the vicinity of the slit2a and a maximum in the vicinity of the slits 2b and 2c. The pole face1c has a shape such that the gap between it and the rotor 4 is alsovariable, there being a minimum at the middle 1d of the pole face 1c andtwo maxima in the vicinity of the slits 2b and 2c, respectively. Thestator 1 has, as can be seen in FIG. 1, an axis of symmetry 7 passingthrough the middle 1d of the pole face 1c, through the axis 3 of therotor 4 and through the middle of the slit 2a.

It is to be noted that the special shape of the pole face 1c results, inconjunction with the magnet of the rotor 4, in the formation of apositioning torque. This torque provides the rotor 4 with two stablebalanced positions, in the absence of any magnetic field other than thatof the magnet itself, which are respectively the two positions in whichthe poles N and S of the magnet are both suituated on the axis ofsymmetry 7.

The stator 1 carries two coils 5 and 6 one of which is located betweenthe pole faces 1a and 1c and the other between pole face 1c, which isthus common to the two coils, and the pole face 1b. When the coils 5 and6 are energized by currents I₅ and I₆, they subject the rotor 4 tomagnetic fields R₅ and R₆, respectively, the directions of which aresubstantially symmetrical with respect to a diameter of the rotor andwhich are mutually inclined, that is to say that the angle between thetwo directions is different from 0° and from 180°. The directions ofthese fields are advantageously mutually inclined at an angle of 90°.The sense of the currents I₅ and I₆ determines in each case the sense ofthe corresponding fields R₅ and R₆.

Four different conditions can occur:

1. When, as represented in FIG. 1, the currents I₅ and I₆ have a sense(which will be referred to later as the positive sense) such that,within the coil 5, the field is directed from the zone of the pole face1c towards the zone of the pole face 1a (arrow 11) and that, within thecoil 6, the field is directed from the zone of the pole face 1b towardsthe zone of the pole face 1c (arrow 12), these currents create outsidethe coils, fields R₅ and R₆ which are respectively directed from thepole face 1a towards the pole face 1c and from the pole face 1c towardsthe pole face 1b. The sense of these fields will also be referred to aspositive. The resultant field R₅₋₆ transverses the zone of the rotor 4,at least to a first approximation, in a direction substantiallyperpendicular to the axis of symmetry 7 and is directed away from thepole face 1a, which plays the role of a North pole (N), towards the poleface 1b, which plays the role of a South pole (S).

2. When, as represented in FIG. 2, the current I₅ has a sense which isinverse with respect to the positive sense as defined hereabove, that isto say when it is negative, the current I₆ still being positive, thefields created by these currents within the coils are directedrespectively as indicated by the arrows 15 and 16. The resulting fieldsR₅ and R₆ outside the coils are consequently respectively directed frompole face 1c towards pole face 1a and from pole face 1c towards poleface 1b. The resultant field R₅₋₆ then passes through the region of therotor 4 in a direction substantially parallel to the axis of symmetry 7and is directed from the pole face 1c, which plays the role of a Northpole (N), towards the pole faces 1a and 1b which together play the roleof a South pole (S).

3. When, as represented in FIG. 3, the currents I₅ and I₆ are bothnegative, thus creating fields R₅ and R₆ directed as indicated by thearrows 9 and 10, the resultant field R₅₋₆ is directed, perpendicular tothe axis of symmetry 7, from the pole face 1b, which thus plays the roleof a North pole (N), towards the pole face 1c which thus plays the roleof a South pole (S).

4. When, finally, as represented in FIG. 4, the current I₅ is positiveand the current I₆ negative, thus creating fields R₅ and R₆ directed asindicated by the arrows 13 and 14, the resultant field R₅₋₆ is parallelto the axis 7 and is directed from the pole faces 1a and 1b, whichtogether play the role of a North pole (N), towards the pole face 1cwhich plays the role of a South pole (S).

Consequently, it is seen that one can create, in the region of therotor, a resultant magnetic field which can adopt any one of fourdifferent directions, depending on the sense of the energising currentspassing through the coils 5 and 6. By suitably commutating the sense ofthese two currents, this field can be rotated in either one direction ofthe other, in order to drive the rotor in the corresponding sense, aswill be seen later.

It will be assumed, to start off with, that the rotor 4 is oriented asindicated in FIG. 1, that is to say with its North pole situated in thevicinity of the pole face 1c. To rotate the rotor 4 clockwise in thesense of the arrow 8, which will be referred to hereafter as thepositive sense, it is sufficient to supply simultaneously to the twocoils 5 and 6 positive currents I₅ and I₆ by means of a suitableelectronic control circuit. The resulting field R₅₋₆ then acts on themagnet of the rotor so that its North pole goes closer to the pole face1b. The torque thus created rotates the rotor in the positive sensesubject to the condition, obviously, that the torque must be greaterthan the total of the positioning torque and of the frictional torqueexerted on the rotor by the mechanical elements which the motor has todrive.

When the rotor 4 has rotated through about 90° and occupiesapproximatively the position represented in FIG. 2, the control circuitreverses the sense of the current I₅, which becomes negative, withoutchanging the sense of the current I₆. Consequently, the field R₅₋₆ isthen directed as indicated on FIG. 2, which creates a fresh torque, ofthe same sense as that mentioned above. As a result the rotor continuesits rotation, still in the positive sense, until it occupies theposition represented in FIG. 3, that is to say the position where itsSouth pole is located in the vicinity of the pole face 1c. Thus, therotor has effected a first step of 180° and the currents I₅ and I₆ canthen be interrupted.

To cause the rotor 4 to effect a second step of 180°, the controlcircuit supplies negative currents into both the coils 5 and 6.Consequently, the resultant field R₅₋₆ has the direction represented inFIG. 3 and thus creates, with the magnet of the rotor 4, a torque whichagain drives this rotor in the positive sense.

When the rotor has rotated for about half a step, the control circuitreverses the current I₅ which becomes positive and the resulting fieldR₅₋₆ adopts the direction represented in FIG. 4. Consequently, the rotor4 continues to rotate in the positive sense and ends its second step on180°. The control circuit then interrupts the currents I₅ and I₆. Thesuccessive pulse forms of these currents are illustrated in FIG. 5a.

In order to cause the rotor to rotate in the opposite sense, referred toherein as negative, from the position represented in FIG. 1, the controlcircuit supplies negative currents into both the coils 5 and 6.Consequently, the field R₅₋₆ takes the sense it has in FIG. 3 and therotor makes a first half-step of 90° in the negative sense. At thismoment, the rotor is in the position represented in FIG. 4 and thecontrol circuit reverses the sense of the current I₆, which thus becomespositive. The field R₅₋₆ is then directed as illustrated in FIG. 2.Consequently, the rotor continues its rotation in the negative senseuntil it has finished its second half-step and occupies the positionillustrated in FIG. 3. The control circuit then interrupts the twocurrents I₅ and I₆.

In order to cause the rotor to make the fresh rotation of one step inthe negative sense, the control circuit supplies both the coils 5 and 6with positive currents I₅ and I₆. Consequently, the field R₅₋₆ takes thedirection it has in FIG. 1 and the rotor rotates half a step in thenegative sense. The control circuit then reverses the sense of thecurrent I₆, which becomes negative, and the field R₅₋₆ takes thedirection it has in FIG. 4. Consequently, the rotor terminates its stepand is again in its starting position. Then the control circuitinterrupts currents I₅ and I₆.

FIG. 5b illustrates the successive pulse forms of these currents.

In FIG. 5a, the graph illustrating currents I₅ and I₆ furtherillustrates the application of a current I₅ by way of a first pulse P1having a first polarity and a second pulse P2 having a second polaritysimultaneous with the application of current I₆ by way of first andsecond pulses P1 and P2 both having the same first polarity. Thereafter,current I₅ is applied by way of a third pulse P3 having the secondpolarity and a fourth pulse P4 having the first polarity simultaneouswith the current I₆ being applied by way of third and fourth pulses P3and P4 both of the second polarity.

In FIG. 5b, current I₅ is illustrated as being applied by first andsecond pulses P1 and P2 both having the second polarity simultaneouswith current I₆ being applied with first pulse P1 of the second polarityand second pulse P2 of the first polarity. Thereafter, current I₅ isapplied by way of third and fourth pulses P3 and P4 both of the firstpolarity simultaneous with current I₆ being applied with a third pulseP3 of the first polarity and a fourth pulse P4 of the second polarity.

FIG. 6 illustrates one example of a practical circuit enabling the coils5 and 6 of the motor to be supplied with the pulses of energizingcurrent represented in FIG. 5.

In this example, these pulses have a period of one second and a durationof 7.8 ms millisecond.

Coils 5 and 6 are each connected to the outputs of two inverters I₁ andI₂, and I₃ and I₄ respectively, each constituted by two complementaryMOS transistors. When the inputs of these inverters are at the samelogic stage, no current circulates in the coils 5 and 6. When the inputof the inverter I₁ (or I₃) is at the logic state 0, while the input ofthe inverter I₂ (or I₄) is at the logic state 1, a current circulates inthe coil 5 (or 6) in the sense indicated by the arrows f.

When the states of the inputs of the inverters I₁ and I₂, I₃ and I₄respectively, are interverted, a current circulates in the correspondingcoil in the reverse sense to that indicated by the arrow f.

A logic circuit C receives from a time base, constituted by anoscillator A and frequency divider B, two signals having respectively afrequency of 1 Hz and 128 Hz. It uses these two signals, as well as asignal S controlling the sense of rotation of the rotor, to furnish ateach second to the inverters I₁ to I₄, which are connected to theoutputs C₁ to C₄, the necessary logic states so that the desiredcurrents circulate in the coils 5 and 6. The logic circuit C will not bedescribed in more detail, since it can readily be designed withoutfurther description by a man skilled in the art in the exercise of anormal design function. It needs merely be noted that the signal at 1 Hzwhich the circuit receives determines the period of the pulses ofcurrent which circulate in the coils, and that the signal at 128 Hzdetermines their duration. As a matter of fact, the period of this lastsignal is equal to 7.8 ms.

The table 7a summarizes the complete operation of the motor. In thistable, the positive currents are designated by the sign + and thenegative currents by the sign -. The column entitled R₅₋₆ gives, foreach combination of currents I₅ and I₆, the sense of the field theycreate in the rotor 4, such as are indicated in FIGS. 1 to 4. The twocolumns "Rotor start" and "Rotor arrival" also indicate, by means ofarrows, the position of start and of arrival of the rotor 4. Thesearrows are directed from the South pole towards the North pole of themagnet of the rotor 4.

The present motor has the important advantage of always rotating in thedesired sense, even if a step has been missed, or if the rotor has madeone step too many. The table of FIG. 7b illustrates a case where, forany reason, the rotor 4 is in the position opposite to that in which itshould have been at the moment corresponding to the first line of thetable. When the control circuit sends the two currents I₅ and I₆ in thepositive sense, the rotor 4 makes half a step in the negative sense.When the sense of the current 5 is reversed, it makes half a step in thepositive sense and is again in its starting position which is preciselythat in which it must be at this moment of the cycle. From this pointon, it rotates in the desired sense. It can easily be seen that therotor resumes, in all cases, the desired sense of rotation in a similarway, whatever this sense of rotation may be and whatever may be themoment of the cycle at which occurs the incident which brings this rotoragain into an erroneous position.

It is obvious that, before being reversed at the end of a firsthalf-step, each of the currents I₅ and I₆ could be interrupted for atime, the inertia of the rotor 4 then resulting in the rotor finishingthis half-step and even starting the second half-step. Similarly, thecurrents I₅ and I₆ could be interrupted before the rotor 4 haseffectively ended its full step. The positioning torque and the inertiaof the rotor would then cause the rotor 4 to end its step. Similarly,the coils 5 and 6 could be short-circuited by the control circuitbetween the rotation steps in order to increase the positioning torqueacting on the rotor and to dampen oscillation of the rotor about itsbalanced position at the end of the steps. The manner of employment ofthese measures, which results in an appreciable saving of energy, mainlydepends upon the construction of the motor and upon the load it has todrive and has to be decided upon at the time of development of the wholearrangement with which the motor is to be associated.

It is still to be mentioned that, due to the fact that the two coils 5and 6 are always fed simultaneously and hence jointly contribute to theformation of the magnetic field creating the torque applied to therotor, the volume of the coils can be substantially reduced as comparedwith that of the prior coils which are alternatively fed; in otherwords, for a given total volume, the torque applied to the rotor can besubstantially increased.

The modification of FIG. 8 distinguishes from the first embodiment bythe fact that the stator is formed of only two coils, designated 17 and18, the coils being constituted by two loop-like coils without anarmature. The rotor 19 is arranged inside the coils 17 and 18 rotatinground an axis 20. This axis is situated in the bissecting plane 21 ofthe median planes 17a and 18a of the two coils 17 and 18, respectively.A positioning element 22, made of magnetically soft material, orientsthe rotor so that, in the balanced position of the rotor, its North andSouth poles lie in the plane 21.

So far as the principle of operation of this modification is concerned,it is absolutely the same as that of the first embodiment.

According to the third embodiment illustrated in FIGS. 9 and 10, themotor comprises a stator the armature of which is formed of two elementsmade of magnetically soft material. One of these elements, designated23, has the shape of a letter E the three branches of which aredesignated 23a, 23b and 23c, respectively. The other element, designated24, has substantially the shape of a rectilinear bar presenting threetransverse protrusions two of which, designated 24a and 24b, aresituated at its ends, and the third of which, designated 24c, issituated at its middle portion. These two elements, 23 and 24, of thearmature of the stator are disposed one with respect to the other in therelative positions, represented in the drawings, that is to say thatthey are positioned opposite one another. The branches 23a, 23b and 23cof the E-shaped element 23 are applied against the protrusions 24a, 24band 24c, respectively, of the element 24 of the said stator. Thisassembly is maintained by two screws 25 one of which passes through thebranch 23a and is threaded into the protrusion 24a, and the second ofwhich passes through the branch 23b and is threaded into the protrusion24b.

A circular hole 26 is provided in the E-shaped element 23, in the regionof the root of the median branch 23c. This hole thus provides threereduced portions 23d, 23e and 23f, each having the shape of an isthmusinterconnecting to each other an adjacent two of three pole faces. Thefirst pole face is constituted by the branch 23c. The two other polefaces are constituted by the portions of the body itself of the element23 situated between the thinner portion 23d and 23e, and 23e and 23frespectively.

The rotor of the motor comprises a shaft 27 pivotally mounted betweentwo elements 28 and 29 of the frame of the apparatus, which may or maynot be of a horological nature, equipped with the present motor. Thisshaft carries a permanent magnet 30 which is bipolar and thediametrically opposed poles of which have been indicated by N and S inFIG. 9.

The stator of the motor as described and illustrated comprises twocoaxial coils 31 and 32 which are wound on the two rectilinear portions24d of the element 24 of the armature. One coil 31 is situated betweenthe protrusion 24a and the protrusion 24c of the element 24 and theother coil 32 is situated between the protrusion 24b and the protrusion24c thereof. The magnetic fields produced by these coils in the armaturehave been represented diagrammatically in FIG. 9 where they aredesignated R₉ and R₁₀.

When they pass through the rotor 30, these two magnetic fields R₉ andR₁₀ are oblique with respect to each other and symmetrical with respectto the diameter of the rotor lying in the plane of the section X--X. Thedirections of these fields advantageously make an angle of 90° withrespect to each other.

Depending upon the sense of the current which passes through the coils31 and 32, the two fields R₉ and R₁₀ could be divergent, as indicated bythe arrows of FIG. 9, in which case the resultant field, which isdiametrical, will coincide with the plane of the section X--X and willbe directed towards the top of FIG. 9. They could also be convergent, inwhich case the resultant field, which is again diametrical, will alsocoincide with the plane of the section X--X, but will now be directedtowards the bottom of FIG. 9. They could also be directed in theopposite senses in which case the resulting torque will be diametrical,but perpendicular to the plane of section X--X in either one sense orthe other.

Hence, by suitably commutating the sense of the two currents passingthrough the two coils 31 and 32, one can selectively cause the resultantfield to rotate in one directional sense or the other, whereby to drivethe rotor in the same sense. More generally, the operation of this thirdform of construction of a motor in accordance with the invention isidentical to that of the first form of construction described herein.

It is to be noted that the fact the magnet 30 is bipolar while thearmature of the stator has three pole faces determines a balanceposition of the rotor which is situated opposite the pole face situatedbetween the isthmus 23d and 23f, that is to say the pole face throughwhich the magnetic flux of the magnet 30 of the rotor follows the pathhaving a minimum reluctance.

In the case of the motor of FIGS. 9 and 10, the two elements 23 and 24of the armature are applied one against each other, these elements beingplaced in different planes as shown more particularly in FIG. 10. Thisarrangement is different in the case of the modification of FIGS. 11 to13 where the two elements of the armature of the stator, now designatedrespectively 33 and 34, are situated in the same plane. Member 33, whichhas the shape of an E, is provided with three branches 33a, 33b and 33cwhile element 34, which is rectilinear, is provided with threeprotrusions 34a, 34b and 34c. The ends of the branches 33a and 33b arenotched at 35, at half-thickness, while the protrusions 34a and 34b arenotched at 36, also at half-thickness. The notched portions are appliedagainst each other and are traversed by securing screws, designated 37,as shown in FIG. 11.

So far as the median branch and protrusion of the two elements 33 and 34of the armature are concerned, they are not notched at half-thicknessbut the protrusion 34c is provided with a semi-circular notch 38 inwhich engages a protrusion of corresponding shape 33d of the medianbranch 33c of element 33 of the armature (FIGS. 11 to 13).

The coils of this modification have not been shown but they areidentical with the coils 31 and 32 of the embodiment of FIGS. 9 and 10and serve to drive the rotor 30 the same way as has been described inconnection with the first form of construction.

Finally, in FIG. 14 there is represented a modification of the armaturesof the motors of FIGS. 9 and 11. According to this modification, notches41, 42 and 43 are provided in the armature element 23 (or 33) in thethinner portions 23d to 23f thereof. The notches 42 and 43 open into thecircular hole 26. These notches serve on the one hand for thepositioning of the rotor and on the other hand for the magneticseparation of the pole faces.

We claim:
 1. A bidirectional stepping motor comprising:a rotor constituted by a permanent magnet which is mounted for rotation round an axis and which provides a permanent magnetic field; a stator including at least two electrical coils, each coil having the shape of a loop and having a median plane, the coils being arranged relative to one another so that their median planes form a dihedral angle, the rotor being arranged within the loops of the electrical coils and the axis of rotation of the rotor being arranged along a bisector of said dihedral angle; and means for independently applying to said coil electrical pulses and for controlling the polarity thereof, said coils producing magnetic fields in response to said pulses and said rotor being subjected to said magnetic fields.
 2. The motor of claim 1, wherein said pulses are applied to said coils for applying simultaneously two magnetic fields to said rotor.
 3. The motor of claim 2, wherein the two directions of said two magnetic fields are inclined at an angle of approximately 90°.
 4. The motor of claim 2 or 3 in which the rotor is rotated by two steps of 180° each in response to pulses having a first or a second polarity wherein the applying and controlling means apply:to the first coil, a first pulse of the first polarity and a second pulse of the second polarity, and to said second coil, first and second pulses both having the first polarity for producing a first step in a first direction of rotation; to the first coil, a third pulse having the second polarity and a fourth pulse having the first polarity, and to said second coil, third and fourth pulses both having the second polarity, for producing the second step in said first direction of rotation; to the first coil, first and second pulses both having the second polarity, and to the second coil, a first pulse having the second polarity and a second pulse having the first polarity for producing a first step in a second direction of rotation; and to the first coil, third and fourth pulses both having the first polarity, and to said second coil, a third pulse having the first polarity and a fourth pulse having the second polarity for producing a second step in the second direction of rotation.
 5. The motor of claim 4 further including a magnetic positioning element located along the bisector of said dihedral angle for maintaining said rotor in a magnetic stable position in the absence of said magnetic fields.
 6. The motor as claimed in claim 5 in which said stator is armatureless. 